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This can be a very useful way of keeping track of email addresses that you hand out to different companies. For instance when signing up to a new web service, rather than giving out your regular email address, you give our companyname@joecitizen.fastmail.fm. If you start getting spam at companyname@joecitizen.fastmail.fm, you can just use the Define Rules screen to block that address.

One problem with this is that technically _’s (underscores) aren’t valid in hostnames/domains (the part to the right of the @ symbol). So if your account was joe_citizen@fastmail.fm, then anything@joe_citizen.fastmail.fm is not technically a valid email address. In many cases it will work, but for strict systems, they might reject the email.

There’s now a work around to this. Simply replace the _ (underscore) with a – (hyphen). Eg use anything@joe-citizen.fastmail.fm. You should only do this for sub-domain addresses where your username/alias has an _ in it. If you’re using the regular joe_citizen@fastmail.fm address, do not replace the _ (underscore) with a – (hypthen).

This is useful when the target is a fastmail account, because the +anything is used to do fuzz folder matching to automatically file the message into a folder.

However if the target address is an external non-fastmail account, then this propagation may actually be annoying since it may result in an invalid email address that you didn’t actually want to send to.